Arbil, Iraq - President Barack Obama has ordered US warplanes back into Iraqi skies to stop jihadists from moving into autonomous Kurdistan and carrying out a potential genocide against displaced minorities.

The US air armada´s first mission was to drop food and water to thousands of people who have been hiding from the Sunni extremist militants in a barren northern mountain range.

Three years after he ended eight years of US occupation, US President Barack Obama announced late Thursday he had authorised limited air strikes against the Islamic State (IS) group.

Many people who have been cowering in the Sinjar mountains for five days in searing heat and with no supplies are Yazidis, a minority that follows a 4,000-year-old faith.

Obama accused the IS, which calls Yazidis "devil-worshippers", of attempting "the systematic destruction of the entire people, which would constitute genocide".

He also justified possible air strikes because of the jihadist threat to Washington´s Kurdish allies, following a lightning advance that saw the Sunni extremists move within striking distance of Arbil.

Panic had begun to grip the Kurdish capital after IS thrust into the Nineveh plains separating their main hub of Mosul and the autonomous territory over the past two days.

"We plan to stand vigilant and take action if they threaten our facilities anywhere in Iraq, including the consulate in Arbil and embassy in Baghdad," Obama said.

Senior US officials, however, denied a Kurdish claim that US warplanes had already struck jihadist targets.

The Kurdish peshmerga, short of ammunition and stretched thin along a huge front, have been forced to retreat in the face of brazen assaults by the jihadists.